Water heating unit with integral thermal energy storage

ABSTRACT

A water heater is provided which includes a heat exchange unit including heat exchange pipes filled with a phase change material. The phase change material has a freezing/melting temperature from about 58° C. to about 62° C. The water heater provides significantly greater thermal energy storage than conventional water heaters, and the excess heating capacity may be used for residential heating applications by using the water heater in combination with a liquid to air heat exchanger.

BACKGROUND OF THE INVENTION

The present invention relates to a hot water heater, and more particularly, to a hot water heater which utilizes phase change materials for the storage and release of thermal energy.

Conventional water heaters provide heated water by storing heat energy in the water as sensible heat. Because the heat energy is stored in the water, a large portion of the heater must be dedicated to storing the heated water until it is used. Typically, water stores approximately 10 calories/gram of sensible heat per 10° C. increments. By requiring a large portion of the heater for storage of heated water, the water heater must be of a reasonable size to store enough heated water for use in a standard residential environment. Typical residential water heaters have a 40 to 60 gallon capacity. For use in commercial buildings and industry, water heaters must be even larger.

Water heaters consume a high percentage of residential energy heating water for bathing, washing dishes, washing clothes and heating homes. In homes heated by electricity, the consumption of electric power is even greater. Overall, a large imbalance in electric power usage exists during daylight hours due primarily to the large amounts of power consumed by industry, businesses and public transportation. To compensate for the extensive day time use of electric power, utility companies provide generating capacity sufficient to supply day time usage, leaving unused capacity available for night hours.

Thus, a need has arisen in the art for a water heater which can more efficiently heat water, which can make effective use of electric power during off-peak hours to minimize building and household power consumption, consequently reducing building and household utility costs.

Water heaters are known which utilize phase change materials to heat water more effectively. Such phase change materials have a latent heat which is greater than the sensible heat of liquid water. A water heater utilizing a phase change material is described in my U.S. Pat. No. 6,493,507. The heater includes heat exchange tubes with water circulating through the tubes and a phase change material surrounding the tubes such that the heat stored in the phase change material can be transferred through the tubes to the water. The phase change material comprises linear alkyl hydrocarbons formed from synthetic, even-numbered carbon chains. However, a disadvantage of such hydrocarbons is that they tend to sublime, i.e., they transform directly from the solid state to the gaseous state when heated, without forming a liquid phase. This limits their application in heat storage applications due to the large change in volume from solid to gas which must be accommodated.

Accordingly, there is still a need in the art for a water heater utilizing a phase change material to effectively heat water which provides improved thermal energy storage, improved heat transfer, and which may be smaller in size than conventional water heaters.

SUMMARY OF THE INVENTION

Embodiments of the invention meet that need by providing a water heater which can more effectively heat water by utilizing phase change materials inside heat exchange pipes which are preferably in cylindrical form. Such a configuration provides improved thermal energy storage and heat transfer.

According to one aspect, a water heater is provided comprising a shell, a cold water inlet for receiving water in the shell, a hot water outlet for transporting heated water from the shell, and at least one heating element in the shell for heating water. Preferably, the water heater includes first and second heating elements which are positioned adjacent to the top and bottom of the water heater, respectively.

The water heater further includes a heat exchange unit in the shell comprising a plurality of heat exchange pipes positioned therein. The heat exchange pipes contain a phase change material therein and are positioned such that the water heated by the heating elements flows around the pipes and heats the phase change material such that the heat stored in the phase change material is transferred through the pipes to the water contained in the shell.

In one embodiment, the phase change material for use in the water heater comprises a linear crystalline alkyl hydrocarbon comprising about 85% by weight of a linear crystalline alkyl hydrocarbon having chain lengths from C₂₀ to C₂₄ and about 15% by weight of a linear crystalline alkyl hydrocarbon having a mixture of carbon chains having an odd number of carbons and carbon chains having an even number of carbons, with the lengths of such chains varying from C₂₂ to C₂₆. The phase change material preferably has a melting/freezing temperature between about 58° C. to 60° C.

In an alternative embodiment, the phase change material comprises stearyl alcohol having a melting/freezing temperature between about 58° C. to 60° C.

The phase change material may further include a number of additives. In one embodiment, the phase change material further includes from about 5 to 10% by weight low density polyethylene. The phase change material may also include from about 1 to 10% by weight carbon black.

In another embodiment, the phase change material may include from about 0.05 to 0.5% by weight of a surfactant. The surfactant preferably comprises a non-ionic surfactant, such as, for example, polyethylene glycol monolaurate.

The phase change material may further optionally include from about 0.05 to 0.5% by weight zinc stearate.

The phase change material is preferably provided in the form of a solid which is molded to fit inside the heat exchange pipes. The pipes are preferably cylindrical in shape.

The heat exchange pipes comprise a metal selected from copper, stainless steel, and glass-coated steel. The heat exchange pipes have an outer diameter of from about 0.5 to about 2.5 inches, and preferably include a closure such as a cap at each end such that the phase change material is sealed therein.

The water heater preferably has a three-dimensional rectangular or cubic configuration with generally flat sides which allows the heat exchange unit to have a similar configuration for placement inside the shell. A layer of insulation may be included on the exterior surface of the shell. The insulation is preferably vacuum panel insulation having an R value of about 50 to 60 per inch of thickness.

In operation, water is supplied to the heater through the cold water inlet and the water is heated from the top down by controlling the temperature of the heating element(s). As the water is heated, heat from the heated water is transferred from the metal heat exchange pipes to the phase change material contained therein. As the phase change material melts, heat is transferred from the phase change material to the water. As the heat is transferred to the water, the temperature of the water is raised and may be transferred out for use through the hot water outlet.

In one embodiment, the water heater may include a solenoid valve in conjunction with the water inlet or outlet to provide pulsatile flow of the water and improve heat transfer.

In yet another embodiment, a home heating system is provided which comprises a water heater as described above and a liquid to air heat exchanger for transferring the heat from the water to air. The liquid to air heat exchanger includes a hot water inlet for receiving heated water from the water heater, and an air inlet and air outlet.

Accordingly, it is a feature of the invention to provide a water heater which employs a heat exchange unit comprising a plurality of heat exchange pipes including a phase change material therein. These, and other features and advantages of the invention, will become apparent from the following drawings, detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A is a cross-sectional view of one embodiment of the water heater;

FIG. 1B is a top view of an embodiment of the heat exchange unit illustrating the arrangement of heat exchange pipes;

FIG. 2 is perspective view of a single heat exchange pipe and cylindrical phase change material; and

FIGS. 3 is a cross-sectional view of the water heater in combination with a liquid to air heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1A and 1B, one embodiment of the water heater 10 is shown. As shown in FIG. 1A, the water heater 10 comprises a shell 12 including a heat exchange unit 14 therein which comprises a plurality of heat exchange pipes 16 including a phase change material 18 therein.

The shell 12 is preferably rectangular or cubic in configuration and should have dimensions sufficient to be able to accommodate up to about 80 gallons of water. The shell 12 may be comprised of a suitable metal such as copper, stainless steel, or glass-coated steel. It should be appreciated that the shell and the heat exchange pipes should preferably comprise the same material. For example, if the heat exchange pipes comprise stainless steel, the shell should also be comprised of stainless steel to provide optimum heat transfer.

As shown, the shell 12 has a generally flat exterior surface 20 which is surrounded by an insulation material 22. The insulation material 22 preferably covers substantially the entire exposed exterior surface 20 of shell 12. Preferably, the insulation material 22 has an “R” value of at least about 50 to 60 per inch. Vacuum panel insulation suitable for use includes vacuum panel insulation commercially available from AcuTemp under the designation ThermoCar®.

Water is supplied to the water heater 10 by means of a water inlet line 24 and is filled close to the top of the shell as indicated by water level 48 as shown in FIG. 1A. A water outlet line 26 allows heated water to flow from the water heater 10 to a source such as a faucet. If desired, one or both of the water outlet lines may include a programmable solenoid valve 50 as shown on water line 26. The solenoid valve may be partially closed at regular intervals to provide pulsatile flow of water to improve heat transfer. The solenoid valve can be programmed to vary both the amplitude and frequency in which the valve is partially closed to provide the desired pressure drop. Variations of frequency from 5 to 60 cycles per minute, and more preferably, from about 15 to 30 cycles per minute are desirable. The valve closure is preferably regulated so as to create a pressure drop of at least 5 psi and preferably up to about 50% of the available water pressure.

The water heater further includes heating elements 28 and 30 positioned adjacent the top and bottom portions of the shell. The heating elements 28, 30 preferably comprise resistance heating elements and are connected to a power supply (not shown). To control the water temperature of the water heated by the heating elements, the water heater may also include one or more thermostats (not shown).

The water heater 10 may also include a timer (not shown) connected to the power supply to control the power usage of the heater during designated time periods, e.g., turning off the power supply during peak power usage hours.

The water heater preferably further includes a sacrificial anode 60 which is preferably placed in the top section of the water heater above resistance heating element 28 which acts as an active acid scavenger, thus increasing the life of the water heater. The sacrificial anode may be comprised of a material such as aluminum or zinc and may be replaced as needed.

The heat exchange unit 14 in the shell includes a plurality of heat exchange pipes 16 including a phase change material 18 therein. As shown, the heat exchange pipes 16 are positioned vertically in the heat exchange unit. The heat exchange pipes are preferably configured in the shell as shown in the top view of the water heater depicted in FIG. 1B and are preferably held in position by a perforated metal screen (not shown) with circular holes therein which fit around the heat exchange pipes to hold the pipes together as a unit.

Prior to being filled with the phase change material, the pipes are hollow and are preferably comprised of a heat conducting material. Preferably, the pipes are formed from copper, stainless steel, or glass-coated steel. The heat exchange pipes include a closure element such as a cap at each end for sealing the phase change material, which will be described in more detail below. While the pipes and phase change material are shown in cylindrical form, it should be appreciated that both the pipes and phase change material may also vary in shape. For example, the pipes and corresponding phase change material may be square or rectangular in shape.

Suitable phase change materials for use in the invention include those having melting/freezing temperatures from about 58° C. to 62° C. The phase change materials store heat energy from the water and provide heat to the water when necessary. Phase change materials may be repeatedly converted between solid and liquid phases to utilize their latent heats of fusion to absorb, store and release heat during such phase conversions. These latent heats of fusion are much greater than the sensible heat capacities of water. For example, in phase change materials, the amount of energy absorbed upon melting or released upon freezing is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of water over an increment of 10° C. Phase change materials can store approximately three to five times more energy than water over the useful temperature range. Thus, by using phase change materials to store heat instead of storing heat in the water, the water heater of the present invention can provide three to five times more heated water than a conventional water heater of the same volumetric capacity. Alternatively, the water heater could be ⅓ to ⅕ the size of a conventional water heater and still provide the same amount of heated water.

Upon melting and freezing, the phase change material absorbs and releases substantially more energy per unit weight than a sensible heat storage material that is heated or cooled over the same temperature range. The phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point. Additionally, the heated water is delivered at a nearly constant temperature which can be selected to be in the temperature range that is comfortable for bathing and other household activities.

One preferred phase change material for use is a blend of a linear crystalline alkyl hydrocarbon comprising 85% by weight of a linear crystalline alkyl hydrocarbon having a carbon chain length of about C₂₀ to C₂₄ and 15% by weight of a linear crystalline alkyl hydrocarbon having a mixture of odd and even carbon chains. The even carbon chain length is from about C₂₂ to C₂₆, and the odd chain length is from about C₂₁ to C₂₇. Suitable phase change materials are commercially available from IGI Wax under the trade names Astorphase® 60 (85%) and Accumelt™ 64 (15%). The two phase change materials are dry blended together in the 85/15 ratio in powder or pellet form and then melted together to provide a uniform composition. The blend of the two different phase change materials has a very high latent heat and prevents subliming which can occur with the use of other linear crystalline alkyl hydrocarbons.

Another preferred phase change material for use comprises stearyl alcohol, which has a crystalline melting temperature of about 60° C. and a latent heat of about 50 to 60 cal/gram. In addition to the high latent storage capacity, this phase change material can also store and release useful amounts of sensible heat.

The phase change material may further include a number of additives such as carbon black to improve the rate of heat transfer. The phase change material may include from about 1 to 30% by weight carbon black, and more preferably, about 1 to 10% by weight carbon black. Preferred for use is an electrically conductive carbon black such as Cabot XC-72R. The phase change material may further include from about 5 to 10% by weight low density polyethylene as a thickening agent to increase the viscosity of the phase change material during the melting phase and reduce the possibility of leakage.

The phase change material may further include from about 0.05 to 0.5% by weight of a surfactant, which prevents the carbon black from separating out when the phase change material is in a liquid state. The surfactant preferably comprises a non-ionic surfactant such as polyethylene glycol 200 monolaurate or polyethylene glycol 400 monolaurate, which are commercially available.

The phase change material may further include from about 0.05 to 0.5% by weight zinc stearate, which aids in preventing adhesion of the phase change material to the heat exchange pipes.

In one method of making the phase change material for insertion into the heat exchange pipes, the phase change material, along with any additives, is dry blended to obtain a uniform powdery blend using, for example, a blender or other mixing device. The blend may then be fed into a heated extruder and formed into a cylinder 18 as illustrated in FIG. 2 which is sized to fit into the heat exchange pipes 16. Alternatively, the phase change material (blended with additives) may be molded into cylinders by an injection molding machine die equipped with multiple cavities for molding cylinders with the desired dimensions.

Referring now to FIG. 2, a single heat exchange pipe 16 for containment of the phase change material 18 is shown which is initially hollow in form and may be formed from metals including, but not limited to, stainless steel, copper, and glass-coated steel. The outer diameter of the pipe may range from about 0.5 to 2.5 inches, more preferably, about 1 to 2 inches, and most preferably about 1.5 inches. The wall thickness of the pipe should be sufficient to withstand normal pressures during operation and is preferably from about 0.030 to 0.125 inches, and more preferably, about 0.060 inches. The pipe 16 is preferably provided in lengths of about 27 inches, while the cylindrical phase change material 18 is about 24 inches in length so as to provide at least about 3 inches of empty space in the pipe once the phase change material has been inserted. This extra space allows for the increase in volume which occurs when the phase change material melts.

Prior to inserting the phase change material 18 in cylindrical form into the pipe, an end cap 40 is applied to one end of the pipe and adhered thereto by soldering, by a high temperature thermosetting adhesive, or by providing mating threads on the pipe and cap.

The empty pipe is then filled with a source of inert gas such as nitrogen or argon such that most of the oxygen in the pipe is purged. The phase change material 18 is then placed into the pipe and a second end cap 42 is then placed over the open end of the pipe 16 and sealed in a conventional manner as described above. The second end cap 42 includes a hole 44 which has been drilled in the center of the cap to allow residual gas inside the heat exchange pipe to be vented as the phase change material melts and expands for the first time.

After placement of the phase change material 18 into the pipe, the phase change material is then run through a heat cycle, i.e., the phase change material is melted from the top down so that it expands in the pipe. This avoids the buildup of high pressure from the expansion of the phase change materials which could potentially cause swelling and/or rupture of the pipes. When the phase change material has completely melted and has reached its peak volume in the pipe, the hole 44 in the end cap 42 is permanently sealed. The cap may be sealed with a threaded metal screw comprised of the same metal as the pipe, or by the use of a high temperature adhesive or soldering (where copper pipes are used).

The initial heating and expansion of the phase change material should take place prior to final assembly of the water heater, i.e., prior to placing the heat exchange unit inside the shell.

The filled heat exchange pipes are then assembled in a compact configuration. In one embodiment, approximately 24 rows with about 24 heat exchange pipes in each row (arranged in a rectangular or square configuration) are assembled. A perforated metal screen (not shown) having openings to accommodate the pipes may be used at the top and bottom of the pipes to maintain them in proper position. A thin metal strip may also be attached to the bundle of pipes to keep the pipes in place. The bundle of pipes including the phase change material therein is then inserted into the shell of the water heater.

By filling the pipes with phase change material in the form of the inner wall of the pipe, the phase change material is in direct heat transfer contact with the inner surfaces of heat exchange pipes 16 so that, during operation, as water surrounds the heat exchange pipes, heat can be efficiently transferred from the phase change material 18 to the water and vice versa.

The thermal energy supplied from the water heater is delivered on a plateau of nearly constant temperature until the latent heat capacity is exhausted. If desired, lower cost off-peak electricity or a green source of energy such as solar photovoltaic or wind driven devices can be used to supply the energy required to “charge” the phase change material, resulting in significant cost savings for consumers.

Referring again to FIG. 1A, the water heater 10 operates in the following manner. Water is supplied to the heater 10 through water inlet line 24 into shell 12. The heating elements 28 and 30 positioned adjacent to the top and bottom portions of the shell are preferably controlled by separate thermostats (not shown) to cause heating/melting of the phase change material from the top down. For example, the upper thermostat is set to a maximum temperature of about 70° C., which is about 10° C. above the 60° C. crystalline melting temperature of the phase change material inside the heat exchange pipes. By melting the phase change material from the top down, the phase change material can expand into the empty space at the top of the heat exchange pipes without a build-up of pressure. As the water is heated, heat from the water is transferred from the metal heat exchange pipes 16 to the phase change material 18 contained therein. As the phase change material melts, it will expand into the free space at the top of the pipes.

When the water heater 10 is not using heating elements 28 and 30, e.g., during peak times of power usage, the phase change material in the heat exchange unit 14 heats the water. When the temperature of the water approaches the freezing/melting point of the phase change material 18, heat is transferred from the phase change material to the water. As the heat is transferred to the water, the temperature of the water is raised. Because the latent heat of the phase change material is greater than the sensible heat of water, phase change material 18 provides a more efficient storage material for storing heat than water alone as in conventional water heaters. Further, the heated water is supplied by water heater 10 at nearly constant temperature equivalent to the freezing point of the phase change material 18. This “plateau” of constant temperature remains until the latent heat capacity of the phase material 18 has been used up (i.e., all of the phase change material re-solidifies). This further differs from conventional water heaters in which heated water is delivered on a thermocline of descending temperature.

Because of the large thermal energy storage capacity of the unit in comparison with conventional hot water heaters, the excess heating capacity may be used for residential heating applications. FIG. 3 illustrates an alternative embodiment, where like elements are indicated by like reference numerals, in which the water heater is connected to an external liquid to air heat exchanger 60 which is capable of supplying the heating requirements of a residential home. As shown, the heat exchanger includes a hot water line 62 from the water heater 10 to the heat exchanger 60 which may include an optional solenoid valve 64. A water return line 66 is included for the return of water back to the water heater. The liquid to air heat exchanger 60 further includes a cold air inlet 68 and warm air outlet 70. As heated water enters the liquid to air heat exchanger 60, the heat from the water is transferred to air which exits through air outlet 70. The heated air may be circulated through a home in a conventional manner.

While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims. 

1. A water heater comprising: a shell, a cold water inlet for receiving water in said shell, and a hot water outlet for transporting heated water from said shell; at least one heating element in said shell for heating said water; and a heat exchange unit in said shell comprising a plurality of heat exchange pipes positioned therein, said pipes containing a phase change material and being positioned such that the water heated by said at least one heating element flows around said pipes and heats said phase change material; and wherein the heat stored in said phase change material is transferred through said pipes to said water.
 2. The water heater of claim 1 including first and second heating elements positioned at the top and bottom of said water heater.
 3. The water heater of claim 1 wherein said phase change material comprises a linear crystalline alkyl hydrocarbon comprising about 85% by weight of a linear crystalline alkyl hydrocarbon having chain lengths from C₂₀ to C₂₄ and about 15% by weight of a linear crystalline alkyl hydrocarbon having a mixture of carbon chains having an odd number of carbons and an even number of carbons having lengths from C₂₂ to C₂₆.
 4. The water heater of claim 3 wherein said phase change material has a melting/freezing temperature between about 58° C. to 62° C.
 5. The water heater of claim 1 wherein said phase change material includes from about 5 to 10% by weight low density polyethylene.
 6. The water heater of claim 1 wherein said phase change material includes from about 1 to 30% by weight carbon black.
 7. The water heater of claim 1 wherein said phase change material includes from about 0.05 to 0.5% by weight of a surfactant.
 8. The water heater of claim 7 wherein said surfactant comprises polyethylene glycol monolaurate.
 9. The water heater of claim 1 wherein said phase change material includes from about 0.05 to 0.5% by weight zinc stearate.
 10. The water heater of claim 1 wherein said phase change material comprises stearyl alcohol.
 11. The water heater of claim 10 wherein said phase change material has a melting/freezing temperature between about 58° C. to 60° C.
 12. The water heater of claim 1 further including a layer of insulation on the exterior surface of said shell.
 13. The water heater of claim 12 wherein said insulation is vacuum panel insulation having an R value of about 50 to 60 per inch of thickness.
 14. The water heater of claim 1 wherein said phase change material is in the form of a molded cylinder.
 15. The water heater of claim 1 wherein said heat exchange pipes comprise a metal selected from copper, stainless steel, and glass-coated steel.
 16. The water heater of claim 1 wherein said heat exchange pipes have an outer diameter of from about 0.5 to about 2.5 inches.
 17. The water heater of claim 1 wherein said heat exchange pipes include a cap at each end such that said phase change material is sealed therein.
 18. The water heater of claim 1 further including a solenoid valve in conjunction with said water inlet or outlet to provide pulsatile flow of said water.
 19. The water heater of claim 1 having a three-dimensional rectangular configuration.
 20. The water heater of claim 1 having a cubic configuration.
 21. A home heating system comprising the water heater of claim 1 and a liquid to air heat exchanger; said liquid to air heat exchanger including a hot water inlet for receiving heated water from said water heater, an air inlet and air outlet. 